Autonomic nervous system: overview !

The automatic wiring that controls heart rate, blood pressure, airways, gut, bladder, pupils and glands, and why so many drugs act on it.

Updated 2026-11-01

The autonomic nervous system controls involuntary functions through two divisions with mostly opposite effects. The sympathetic division prepares the body for stress («fight or flight»); the parasympathetic division supports rest and digestion. Each uses a two-neuron chain with a ganglion in between, and drugs that mimic or block their transmitters (acetylcholine and noradrenaline) are among the most used in medicine.

Layout

Sympathetic: short preganglionic fibres to paravertebral chain or prevertebral ganglia; long postganglionic fibres to the organs
Parasympathetic: long preganglionic fibres to terminal or intramural ganglia near or in the organ; short postganglionic fibres
Adrenal medulla: receives preganglionic sympathetic fibres directly and secretes catecholamines as hormones

Organ effects at a glance

Heart: sympathetic raises rate and force (beta-1); parasympathetic lowers rate (M2)
Airways: sympathetic dilates (beta-2); parasympathetic constricts and increases secretions (M3)
Eye: sympathetic dilates the pupil (alpha-1, radial muscle); parasympathetic constricts it and contracts the ciliary muscle for near vision (M3)
Gut: parasympathetic increases motility and secretion; sympathetic reduces them
Bladder: parasympathetic contracts the detrusor (M3); sympathetic relaxes it (beta-3) and contracts the internal sphincter (alpha-1)
Blood vessels: mostly sympathetic; alpha-1 constricts, beta-2 dilates vessels in skeletal muscle

Drug groups that act here

Sympathomimetics: adrenaline, noradrenaline, phenylephrine, salbutamol
Sympatholytics: alpha blockers, beta blockers, central alpha-2 agonists such as clonidine
Parasympathomimetics: pilocarpine, bethanechol, cholinesterase inhibitors
Antimuscarinics: atropine, ipratropium, oxybutynin
Ganglion and neuromuscular drugs: see the neuromuscular junction page

Predicting effects

If you know which division dominates an organ at rest, you can predict a drug's effect. Blocking muscarinic receptors removes vagal tone and causes tachycardia, dry mouth, urinary retention and dilated pupils. Blocking beta receptors removes sympathetic drive to the heart and kidney (renin) and can tighten airways in asthma.

Test yourself

Think of the answer, then tap the question to see it.

Which transmitter and receptor are used in all autonomic ganglia?

Acetylcholine acting on nicotinic receptors, in both sympathetic and parasympathetic ganglia.

What is the main exception to noradrenaline release by sympathetic postganglionic fibres?

Fibres to sweat glands (and some skeletal muscle vessels) release acetylcholine, which acts on muscarinic receptors.

Why is the adrenal medulla called a modified sympathetic ganglion?

It receives preganglionic sympathetic fibres directly and its cells release adrenaline and noradrenaline into the blood instead of having axons.

What happens to the heart rate when muscarinic receptors are blocked at rest, and why?

It rises, because resting heart rate is held down by vagal (parasympathetic) tone acting on M2 receptors.

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Example drugs

← All of pharmacology

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: 15.1 Divisions of the Autonomic Nervous System (OpenStax (Rice University), 2022); 15.4 Drugs that Affect the Autonomic System (OpenStax (Rice University), 2022).

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Representative model, for educational purposes only.